A lithium ion secondary battery has been widely used for information-related mobile communication electronic equipment such as mobile phones and laptop personal computers since it enables the battery to become smaller in size and lighter in weight as a battery capable of attaining higher voltage and higher energy density compared to the conventional nickel-cadmium battery and nickel metal hydride battery. With regard to the lithium ion secondary battery, it is thought that the opportunity of being utilized for onboard use in which the battery is incorporated into electric vehicles, hybrid electric vehicles and the like as a means for solving an environmental problem or industrial use such as electric power tools will further increase in the future, and attaining further highly enhanced capacity and highly enhanced output has been eagerly desired.
The lithium ion secondary battery is composed of positive and negative electrodes having at least an active material capable of reversibly inserting/extracting lithium ions and a separator which is arranged in a container and separates the positive electrode from the negative electrode, the container being filled with a non-aqueous electrolytic solution.
The positive electrode is prepared by applying an electrode agent containing a positive electrode active material for a lithium ion battery (hereinafter, sometimes referred to a positive electrode active material or an active material), a conductive additive and a binding agent onto a metal foil current collector made of aluminum and the like and subjecting it to pressure forming. As the current positive electrode active material, a powder of composite oxides of lithium and a transition metal (hereinafter, sometimes referred to as lithium metal oxides) such as lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), or a ternary system material in which a portion of cobalt is substituted with nickel and manganese (LiMnxNiyCo1-x-yO2), and spinel type lithium manganate (LiMn2O4) has been used relatively frequently. Since these materials contain a so-called rare earth element, there is a problem in terms of cost and stable supply. In recent years, olivine-based materials (phosphate-based materials) with a high level of safety have been attracting attention, and above all, lithium iron phosphate (LiFePO4) containing iron which is one of the abundant resources and is inexpensive has begun to be put into practical use, and moreover, lithium manganese phosphate (LiMnPO4) with a high level of output energy has also been attracting attention as a next-generation active material. Separately, metal oxides such as V2O5, metallic compounds such as TiS2, MoS2 and NbSe2, and the like have also been utilized.
Moreover, the negative electrode is prepared, as with the positive electrode, by applying an electrode agent containing an active material, a conductive additive and a binder agent onto a metal foil current collector made of copper and the like and subjecting it to pressure forming, and in general, as the active material for the negative electrode, lithium metal, lithium alloys such as a Li—Al alloy and Li—Sn, silicon compounds in which SiO, SiC, SiOC and the like are the basic constituent elements, conductive polymers prepared by doping lithium into polyacetylene, polypyrrole and the like, intercalation compounds prepared by allowing lithium ions to be incorporated into crystals, carbon materials such as natural graphite, artificial graphite and hard carbon, and the like have been used.
In the active materials currently put into practical use, a theoretical capacity of the positive electrode is far lower than that of the negative electrode, and hence it is indispensable to improve a capacity density of the positive electrode for increasing a capacity of the lithium ion battery. In recent years, for example an olivine-based positive electrode active material, a solid solution-based active material and the like, many active materials are investigated, which have not been put into practical use regardless of their high capacity since the conductivity thereof is low. In order to put these positive electrode active materials into practical use, a technology of imparting electrical conductivity to the positive electrode is required.
In order to improve the electron conductivity in the positive electrode, a technique of adding a conductive additive is employed. Examples of materials heretofore used as the conductive additive include graphite, acetylene black, Ketjen Black and the like. However, particularly, in the positive electrode active material having low electrical conductivity, it is insufficient only to add the conductive additive, and it requires a technique of directly forming an active material/conductive carbon material composite.
Patent Document 1 discloses a technique of coating the positive electrode active material with carbon. Further, Patent Document 2 and Non-Patent Document 1 disclose a technique in which a graphene oxide and a positive electrode active material are mixed and then the resulting mixture is reduced. Non-Patent Document 2 disclose a technique in which a positive electrode active material is synthesized in the presence of a graphene oxide, and then reduced. Patent Document 3 and Patent Document 4 disclose a technique in which a positive electrode paste including a graphene oxide is applied onto a current collector and dried, and then the graphene oxide is thermally reduced.